There are known in the prior art various high velocity dryers for drying the web formed in the course of making paper on a paper-making machine. In one such high velocity dryer, the web to be dried is passed over a dryer cylinder, the major portion of the periphery of which is enclosed by a hood. Hot drying air is supplied to the hood and is directed against the surface of the web and is then removed from the hood. Most of the systems are recirculating systems in which a certain portion of the air removed from the hood is returned thereto together with an amount of fresh air heated to supply air temperature.
High velocity drying equipment used in the tissue and toweling industries, operates at supply air temperatures as high as 900.degree. F. The design of the supply systems of these dryers has been pointed toward minimizing the exhaust volumes in order to produce the highest drying capabilities at the lowest fresh air heating requirements. As a result the total energy consumption of these systems has decreased substantially. These systems now operate at exhaust humidities of approximately 0.4 pounds of water per pound of dry air, which is a reasonable minimum energy consumption.
The fresh air requirements when a system, such as a Yankee dryer, is operating at a low exhaust volume consists primarily of the air required by the burners to provide oxygen for combustion and the infiltration air required around the periphery of the dryer. This infiltration air is necessary as it is practically impossible to perfectly balance the dryer in such a way as to prevent the intrusion of the hot moist air used for drying into the working area surrounding the dryer. To prevent such intrusion the drying system is run so that air from the area surrounding the hood is drawn into the drying system along the periphery. While the quantity of the cold air infiltrated is rather small, the energy required to heat this air from room temperature to exhaust temperatures represents approximately 15% of the total energy consumption of the system.
In addition, the infiltrated air often results in edge effects from cold air entering the dryer causing uneven drying. Because this infiltration air is spread along the entire periphery of the drying system, it is impossible to eliminate this heat load by means of heat recovery equipment.